Many technologies, such as radio immunoassay (RIA or IRMA: immunoradiometric assay) and enzyme-antibody technique, have been proposed and employed for immunization analysis. According to radio immunoassay, a competitive antigen or an antibody is marked using a radionuclide, and a fixed quantity of the antigen is measured, based on the results of a specific activity. For this method, high sensitivity is an advantage; however, there is a problem as regards the security of the radionuclide, and therefore, a special facility and a special apparatus are required. Further, compared with radio immunoassay, enzyme-antibody technique, which employs an enzyme that is used to mark an antibody, is easier operationally, and satisfies requirements for practical sensitivity; however, there is a continuing demand for more sensitivity and increased operational usability.
Under these circumstances, recently, D. L. Graham, et al., Biosensors & Bioelectronics 18, 483 (2003) proposed a method whereby, in order to easily detect a target material, magnetic particles coupled to the target material are detected by employing a magnetoresistive effect film.
According to the technique disclosed in this reference document, two GMR (Giant Magnetic Resistance effect) films of 2 μm×6 μm are employed to detect magnetic particles having a diameter of 2 μm. Bithion is coupled with the surface of one of the GMR films, to immobilize magnetic particles, but is not coupled with the surface of the other GMR film. Further, the magnetic particles are avidin-modified. Since avidin and bithion are very strongly coupled, magnetic particles are immobilized on one of the GMR films, while on the other GMR film, none are immobilized. Since the magnetized state of the GMR film whereat the magnetic particles are immobilized is changed by the effect produced by a floating magnetic field generated by a magnetic particle, the resistance of this GMR film differs from the other GMR film, whereon no magnetic particles are immobilized. For the GMR films, a multi-layer structure, comprising two magnetic films, between which a non-magnetic metal film is formed, is employed as a basic structure. The resistance value depends on the relative magnetization directions of the two magnetic films, and characteristically, when the magnetization directions are parallel there is comparatively little resistance, but when the magnetization directions are antiparallel there is a comparatively large resistance. In order to provide the parallel magnetized state and the antiparallel magnetized state, the two GMR magnetic films are formed of a magnetic material having a coercive force, such that inversion of the magnetization direction of one of the magnetic films is difficult, and inversion of the magnetization direction of the other magnetic film is enabled by a floating magnetic field generated by a magnetic particle.
When a magnetic field is applied to magnetic particles and the GMR films in the in-plane direction, and when a magnetization direction 902 of the magnetic particles is directed toward the applied magnetic field, as shown in FIG. 9, a floating magnetic field 904 generated by a magnetic particle 901 is applied to a GMR film 905 in a direction opposite to that in which a magnetic field 903 is to be applied. Therefore, the magnetization directions of the two magnetic films composing the GMR film do not become parallel. On the other hand, since the magnetic films of a GMR film whereon magnetic particles are not immobilized are not affected by the floating magnetic field, the magnetization directions are parallel across the entire film. That is, since the magnetized states of the two GMR films differ, a further difference occurs between the resistances, so that the detection of magnetic particles is enabled.
As described above, the magnetic particle detection method employing the GMR films is performed by magnetizing magnetic particles in a desired direction, and by changing the magnetization direction of a magnetoresistive effect film using a floating magnetic field generated by magnetic particles. Using this method, magnetic particles can easily be detected.
In the description of the previous reference document, a detection signal for a magnetic particle obtained by one GMR film differs from that obtained by another, and depends on the number of magnetic particles. This occurs because the size of the area on GMR film that is influenced by a floating magnetic field, generated by a magnetic particle, differs in accordance with the number of magnetic particles. However, when relative to the total area of the GMR film the ratio of the area influenced by a floating magnetic field, generated by a magnetic field, is considerably smaller, a remarkably weak detection signal is obtained, and the detection of magnetic particles is impossible. Therefore, in order to obtain a strong detection signal, the size of the GMR film should be consonant with the sizes of the magnetic particles.
At present, of the commonly employed magnetic particles, a small magnetic particle size is several tens of nm in diameter. In order to detect such a small magnetic particle, it is preferable that a GMR film of the same size, i.e., about several tens of nm, be employed, while taking into account the area affected by a floating magnetic field generated by a magnetic particle. However, when the size of the magnetic film is reduced, the inversion of the magnetization direction becomes more difficult, and the detection signal produced by a magnetic particle is weakened.
Furthermore, since means required to apply a uniform magnetic field to an area for the detection of a magnetic material must be large, the production of a compact sensor is not easy.